We describe a novel biochemical sensing method and its potential new biosensing applications. A light-sensitive complementary metal oxide semiconductor (CMOS) chip prepared through a standard 0.5-μm CMOS process was developed for measuring biochemical reactions. A light producing enzymatic reaction catalyzed by horseradish peroxidase (HRP) was designed as a platform reaction to determine the concentration of hydrogen peroxide (H ₂ O ₂ ) by the CMOS chip with a standard semiconductor parameter analyzer (HP4145). The kinetics of enzymatic reaction were determined and compared with a standard and sophisticated fluorometer (Hitachi F-4500) in a biochemical laboratory. Similar results were obtained by both instruments. Using glucose oxidase as an example, we further demonstrated that the HRP platform can be used to determine other H ₂ O ₂ producing reactions with the CMOS system. The result points to an important application of the CMOS chip in biological measurements and in diagnosis of various health factors.